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RP (Repeated Pairs), a measure of actual or total frequency of repetitions of surnames in different married couples, and RPr, an estimate of the random or expected component of RP, have been calculated for mar­riages registered on the Island of Sanday in 1855-1965 inclusive. RP is higher for residents (at time of marriage) within a single parish than for the residents of the island of Sanday as a whole and higher when both are residents than when one or both partners are non-residents of the island.The values of RP tend to be higher than of RPr (the random component) in a majority of tests. Differences and variations within this study are small compared with the range in RP seen for other populations (e.g. between an isolate on a Caribbean island and an urban area in England). At least in relative terms an element of the genetic structure is apparently evident through this measure of lineage preferences in marriages.